In the manufacture of paperboard products such as multi-ply paper and corrugated board for boxes, containers, and the like, it is desired and preferred that the longitudinal tension forces imparted to the paper web be uniform throughout the width of the web. Absence of uniform tension transversely of the paper web creates paper products which may not be acceptable to the trade and which may require additional handling and processing before being acceptable. Non-uniform tension transversely of a paper web may result in warping of the web, that is the forming of a longitudinally curved paper web instead of a web which maintains planar dimensional characteristics.
It will be understood that the paper stock rolls mentioned above may be made of different weights and thicknesses. Such paper roll stock may have a diameter of 60 inches and a width of up to 100 inches. Such paper stock roll may include a core of about 4 inches in diameter. When the paper web is withdrawn from such a paper roll at speeds up to 700 feet per minute, it will be apparent that inertia forces acting on the rotating roll will be continually changing because of the continual reduction in diameter of the roll. Further, to controllably withdraw a paper web from such a paper stock roll without causing over-running, over-turning or free wheeling of the roll requires the application of braking forces on the roll to prevent such over-turning or free wheeling and also to provide control of longitudinal tension in the paper web. With the introduction of automatic systems for controlling the forming and making of paper products, it has become necessary to more precisely control not only the tension in the paper web so that it is uniform throughout the width of the web, but also to assure that a paper stock roll may be braked to a complete stop within a very limited period, such as less than three seconds, in order to splice a paper web from a different stock roll as may be required by the production operation. Further, in the manufacture of corrugated paperboard, non-uniform tension of the paper stock, from which flutes or corrugations are formed, causes flutes of varying height and form which may not be properly bonded to the facing web. Such irregularly fluted paperboard may not withstand required tests and would be unacceptable.
Precise control of tension in a paper web significantly affects the production of paperboard. Lack of proper tension control may result in end to end warp, wrinkling, misalignment of the web, loss of caliper in corrugated board, malformation of flutes or corrugations, undesirable changes in take-up ratio in the production of single face corrugated board, and lack of control of consumption of material in making corrugated board.
Prior proposed systems for controlling the tension in the paper web and braking the paper stock roll at the roll stand have included the use of hydraulically operated drum and brake shoe braking means. A drum and brake shoe brake means was mounted on the ends of roll stand arms which carry and support the paper stock roll. Each pair of brake means was hydraulically operated through a separate master cylinder. To maintain necessary control of tension, the brake setting on each side of the paper roll was set at about 500 psi, approximately 18 psi being required to overcome the brake shoe release spring forces. In operation at such high pressure settings, the brake shoes and the drums as well as the hydraulic fluid became hot because the brake is applied at such 500 psi during the entire operational time for a paper roll. Increase in temperature of the fluid caused further pressure increase because the heated fluid expanded and the master cylinders were preset to a selected pressure. Thus from time to time during operation, an operator was required to check the pressure settings of the master cylinder at each pair of brake means for a paper roll and to readjust the settings. Because of the mechanical features of such a drum type brake system, pressure variations constantly occured with the result that the tension in the paper web continually varied. Under most careful operator control it was extremely difficult to maintain uniform tension across the width of the paper web.
In another proposed braking system, automatic tension control was attempted by employing electronic control units which calculated the required theoretical brake force pressure on the roll in order to maintain constant tension in the web for a specific roll diameter and paper weight. Such a control unit was coupled electrically and pneumatically to a servomechanism unit which regulated the roll stand brake force by changing pressure in the brake line. Such an electrical control unit was utilized with any type of brake system. Since the braking system still included the mechanical characteristics described above, pressures for which the braking means were set were still in a high range in the nature of 1,000 psi.
Other proposed braking systems included tension control means which were responsive to change in weight of the paper stock roll as the roll was depleted.
In all of the prior proposed tension control means known to me, the numerous variable factors affecting the tension of the paper web were attempted to be regulated under conditions in which the brake pressure setting was relatively high, that is in the nature of from 350 to 1,000 psi.